Method for making a crosslinkable aqueous solution which is useful to form soft, water-swellable polyacrylate articles

ABSTRACT

Water swellable polyacrylate articles are made from a solution of the polyacrylate having an effective amount of a soluble crosslinking agent therein by heating and/or drying the solution. The polyacrylate solution is made from a polyacrylate by saponification and the cross-linking agent is then added.

United States Patent 11 1 Gross et a1.

1 Dec. 16, 1975 [73] Assignee: The Dow'Chemical Company,

Midland, Mich.

22 Filed: -Mar. 13,1974

21 Appl. No.: 450,650

[52] US. Cl ..260/29.6 E; 260/296 TA, 260/78.5 R; 260/89.5 S, 260/296 SO[51] Int. Cl. C08L 31/02; CO8L 33/02 [58] Field of Search 260/785 R,29.6 TA, 29.6 H, 260/296 E, 89.5 S, 29.6 SQ

[ 56] References Cited UNITED STATES PATENTS 2,649,439 8/1953 Brown260/895 S 3,245,933 4/1966 Muskrat 60/296 N 3,514,419 5/1970 Darlow etall. 260/296 H 3,669,103 6/1972 Harper et a1 128/156 3,810,468 5/1974Harper et a1 128/156 Primary Examiner.l0seph L. Schofer AssistantE.\'aminerPeter F. Kulkosky Attorney, Agent, or Firm-Benjamin G. Colley-[57] ABSTRACT Water swellable polyacrylate articles are made from asolution of the polyacrylate having an effective amount of a solublecrosslinking agent therein by heating and/or drying the solution. Thepolyacrylate solution is made from a polyacrylate by saponification andthe cross-linking agent is then added.

9 Claims, No Drawings METHOD FOR MAKING A CROSSLINKABLE AQUEOUS SOLUTIONWHICH IS USEFUL TO FORM SOFT, WATER-SWELLABLE POLYACRYLATE ARTICLESBACKGROUND OF THE INVENTION This invention relates to soft,water-swellable articles made from crosslinked polyacrylates, methodsfor their preparation, and to an aqueous solution of polyacrylates whichis useful to make absorbent articles.

It is known from US. Pat. Nos. 3,669,103 and 3,670,731 that polymericsorbents that are crosslinked during polymerization can be used to makedisposable diapers, dressings and the like.

It is further known from Ser. No. 371,909, filed June 20, 1973, thatwater swellable articles can be made from post-polymerizationcrosslinked polyelectrolytes. However, these articles are generallystiff and brittle and generally require a plasticizer to make a film orcasting soft enough to be useful in most applications, expecially indiapers. A disadvantage of using a plasticizer is that the mosteffective plasticizers are also humectants and make the polyelectrolytearticle very sensitive to high humidity.

For example, if polyacrylates are converted through alkaline esterhydrolysis to the corresponding copolymer of the acrylate and acrylicacid salt, as in Ser. No. 371,909, approximately 80 percent or more ofthe acrylate mers must be so converted before the polymer becomes watersoluble. This is attributed to alkaline attack on the polymer particlefrom the outside in, whereby the polymer molecules on the outside becomemuch more altered by ester hydrolysis than those on the inside of theparticle. At the high degrees of hydrolysis then necessary to solubliizeall the polymer, the resulting polymer is more akin to a salt ofpolyacrylic acid than to a polyacrylate. The softness of the originalpolyacrylate has been replaced by the glassy brittleness of a salt. Thisbrittleness is highly undesirable in water swellable (absorbent)articles.

SUMMARY OF THE INVENTION It has been discovered that the use ofplasticizers can be eliminated in the known post-polymerizationcrosslinked polyacrylates by providing for the partial saponification ofthe alkyl acrylate mers in the polymer chains. Thus, when about 30 toabout 70 weight percent of the alkyl acrylate mers are left intact inthe polymer, the resulting crosslinked article formed from a solution ofthe partially saponified polyacrylate is soft and flexible without thenecessity of adding a special water-soluble plasticizer such asglycerine.

The invention comprises methods of making crosslinkable aqueoussolutions, methods of making soft, water-swellable crosslinkedpolyacrylate films and fibers as well as methods of making an absorbentarticle coated with a dry water-swellable soft polyacrylate and theproducts produced by the above methods.

Before the final products can be prepared, it is essential to'prepare asolution of the polyacrylate wherein an alkyl acrylate, or a mixturethereof with an alkyl methacrylate, is partially saponified with analkali metal hydroxide solution. In general, the method of making thissolution comprises A. forming a polyacrylate solution having about 30 toabout 70 weight percent alkali metal carboxylates by dissolving in anaqueous alkali metal hydroxide solution a polyacrylate comprising 1.about 30 to about 92 percent by weight of an alkyl acrylate wherein thealkyl group has 1-10 carbon atoms, an alkyl methacrylate wherein thealkyl group has 4-10 carbon atoms, or mixtures thereoof; 2. about 8 toabout percent by weight of an olefinically unsaturated carboxylic acid,and 3. about 0 to about 15 percent by weight of an omega hydroxy-alkylacrylate having 14 carbon atoms in the hydroxy alkyl group; I B. heatingthe solution until saponification is complete,

and C. adding to said solution about 0.1 to about 10 weight percentbased on the dissolved polymer of a watersoluble crosslinking agentwhich is reactive with carboxylate salt groups.

The final products of this invention are made by coating a substratewith the above solution or extruding the solution into non-solvents toform fibers both followed by heating and crosslinking the polyacrylates.

The final products of this invention are water-swellable and are usefulwherever aqueous solutions need to be absorbed. The present invention isparticularly useful to make disposable absorbent articles as an additiveto increase absorbency while decreasing the articles bulk. Examples ofthe diverse utilities are surgical sponges, tampons, diapers, meattrays, litter mats for household animals and the like.

DETAILED DESCRIPTION The alkali-soluble polyacrylates useful to form thecrosslinkable solutions of this invention can be made by knowntechniques such as emulsion, suspension, bulk, or solutionpolymerization techniques, so long as they consist of about 50 to 92percent by weight of an alkyl acrylate with 1-10 carbon atoms in thealkyl moiety or an alkyl methacrylate with 410 carbon atoms in the alkylmoiety, about 8 to about 50 perccent by weight of an olefinicallyunsaturated carboxylic acid and about 0 to about -15 percent by weightof an omega hydroxy alkyl acrylate having 1-4 carbons in the alkylmoiety.

It is preferred to use alkali-soluble latices having about 15 to about60 weight percent of non-volatile polymer solids as is set forth below.

Examples of useful alkyl acrylates are methyl acrylate, ethyl acrylate,propyl acrylate, hexyl acrylate and the like. Examples of useful alkylmethacrylates are butyl methacrylates, hexyl methacrylates, octylmethacrylate, decyl methacrylate and the like.

Examples of useful omega hydroxy alkyl acrylates are 2-hydroxyethylacrylate, hydroxymethyl acrylate, 3- hydroxy-propyl acrylate and4-hydroxybutyl acrylate.

The foregoing polyacrylates are then dissolved in an aqueous alkalimetal hydroxide solution. Generally the equivalents of hydroxidesolution used are from about 30 to about 70 percent based on the molarconcentration of polymerized monomer and the preferred amount is fromabout 40 to about 55 percent. In any event, the amount of hydroxidesolution added is sufficient to convert or saponify some of the acrylateesters to alkali metal carboxylates and to neutralize the carboxylicgroups of the polyacrylate used into alkali metal carboxylates so thatthe converted polyacrylate has about 30 to about 70 weight percent ofalkali metal carboxylates.

This solution is then rendered crosslinkable by adding about 0.1 toabout weight percent based on the dissolved polymer of a water solublecrosslinking agent which is reactive or crosslinkable with carboxylatesalt groups.

The foregoing aqueous solution can be used per se to prepare the finalproducts of this invention. However, it is sometimes advantageous to addvarious amounts of volatile monohydric alcohols and/or ketones tdcontrol the viscosity of the aqueous solutions and increase the rate ofdrying. Examples of useful monohydric alcohols are lower alkanols suchas methyl alcohol, ethyl alcohol and propyl alcohol. Examples of usefulketones are acetone, and methyl ethyl ketone.

Illustrative examples of the soluble crosslinking agents useful in thisinvention are polyhaloalkanols such as 1,3-dichloroisopropanol,1,3-dibromoisopropanol; sulfonium zwitte rions such as thetetrahydrothiophene adduct of novolac resins; haloepoxyalkanes such asepichlorohydrin, epibromohydrin, 2-methyl epichlorohydrin andepiiodohydrin; polyglycidyl ethers such as glycerine diglycidyl ether,ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether,diethylene glycol diglycidyl ether; and the mixtures of the foregoing.

The olefinically unsaturated carboxylic acids used in this invention canbe mono or poly carboxylic acids.

Examples of the mono-olefinic monocarboxylic acids are acrylic,methacrylic, crotonic, isocrotonic, angelic, tiglic, senecioic ormixtures thereof.

Examples of the mono-olefinic polycarboxylic acids are maleic, fumaric,itaconic, aconitic, teraconic, citraconic, mesaconic, glutaconic.

Compounds containing two or more of the functional groups of theforegoing crosslinking agents would also be useful, as well asprecursors which would form these functional groups under the conditionsencountered in heating or drying the polyelectrolyte solutions.

Sulfonium zwitterions are known from U.S. Pat. No. 3,660,431, dated May2, 1972; Ser. Nos. 205,754 and 205,755 both filed on Dec. 7, 1971. Thedisclosures of this patent and patent applications are incorporatedherein by reference.

The crosslinking technique used in this invention to transformwater-soluble polyacrylates into insoluble but water-swellable polymersis known as nucleophilic displacement on saturated carbon.

The carboxylate ion on the polyacrylate acts as the nucleophile whilethe cross-linking agent is the substrate for the nucleophilic attack.Typical leaving groups and their corresponding substrates are listed inTable I. Any combination of two or more of these leaving groups on thesame substrate could act as a crosslinking agent for the polyacrylatesof this invention.

Table II lists several illustrative compounds used as cross-linkingagents in this invention and the operable limits to obtain insoluble buthighly swellable polyacrylates according to this invention. Onceinsolubility is reached, higher levels of cross-linker give polymerswhich swell in aqueous media to produce firmer, less slippery gels butof lower actual absorbency.

TABLE I Leaving Groups in Nucleophilic Displacement (R alkyl or aryl)TABLE I-continued Leaving Groups in Nucleophilic Displacement R ulkyl oraryl) TABLE II Cross-linking Agents for Polycarboxylates Crosslinker Wt.of Polyelectrolyte l.3-dichloroisopropanol 04-10% epibromohydrin 05-10%epichlorohydrin l-l0% glycerine diglycidyl ether 0.10-457r novolacsulfonium ion l-l0% The rate of nucleophilic displacement isconcentation dependent and is a factor in this invention. In solution,when the concentration of the cross-linker is very low, the rate ofreaction is quite slow (pot life 10-48 hours before gelation). Once thesolution is applied to a substrate Surface and evaporation of solventbegins, the rate of cross-linking accelerates. Applying heat at thistime increases the reaction rate even more.

If the cross-linking reaction is allowed to proceed in the originalsolution as by heating, aging, or excessive amounts of cross-linker, theabsorbent articles of this invention cannot be fabricated. The solutionwill become progressively more viscous and stringy until it forms acontinuous gel which could not be spread,

sprayed or spun.

In the method of making water-swellable films by the present inventionthe above solution of the polyacrylates is spread on a flat plate orroller of metal, plastic, or other impervious substrate and heated to atemperature greater than 30C. to crosslink the polyacrylate and driveoff the excess water and/or alcohol. The film is then peeled off theplate or roller by a scraper to recover the intact film for subsequentstorage or use.

Similarly, when an absorbent article is prepared, the article which isto be the substrate is coated with the solution of the polyacrylate andthen the coating is cross-linked. It is to be understood that for thepurposes of this invention the coating step implies a complete coatingor a discontinuous coating, thus when a fibrous substrate such ascellulose batting, paper, woven or non-woven cloth, and the like areusedas the substrate, the solution can be applied in a discontinuousmanner, i.e. in a pattern of large dots, squares, or grid lines toretain the inherent flexibility of the fiberous substrate and at thesame time vastly improve its water absorbency. Wood pulp can be coatedby slurrying it in the polyacrylate solution followed by a fluffingoperation.

If desired. the water swellable film prepared as above can be used perse as the inner absorbentlaye'r in baby diapers. It is sometimesadvantageous that the film be disintegrated into flakes, stripso'rpowders. This is accomplished by crushing or comminut'ing the film in ahammer mill, blenders, or the likejlf long flat strips are desired, thefilm can be sliced widthwise with appropriate slicers. I

In some instances, water-swellable fibers are desired. These can beprepared by extruding the above solution of the polyacrylates into abath comprising lower alkyl ketones such as acetone, methyl ethylketone, diethyl ketone and the like. Alcoholic solutions may be extrudedinto a non-aqueous coagulant such as chlorinated hydrocarbons, i.e.methylene chloride, perchloroethylene and the like. The soft extrudedfibers are then removed from the bath by any convenient means such as athree or five roll cluster and carried through a heated chamber at atemperature greater than about 30C. and preferably in the range fromabout 70 to about 150C. to dry and to crosslink the polyacrylate fibers.

The absorbency of the crosslinked polyacrylates (grams of solutiongelled per gram of pol-yacirylate) is determined in the following mannerusing synthetic urine (0.27 N sodium chloride solution).

'A 0.5 gram sample of a crosslinked polyacrylate is weighed into a 250ml. beaker, a 0.27 N sodium chloride solution (150 ml.) is poured intothe beaker and allowed to soak for 2 hours at room temperature, withoccasional'sti'rring. The swelled polyacrylate is then collected byfiltration and the gel capacity is reported as grams of solution gelledper gram of polymer salt.

Thefollowing examples are presented solely to illustrate but-'not limitthe invention.

EXAMPLE 1 Three mixtures were made up having the following compositions.

' Part B 2.1 g. Sodium 28 g. Methacrylic Persulfate Acid 8.4 g. AcrylicCld 3. l g. t-Dodecyl Merca ptan "dioctylsodiurn sulfosuccinate (TritonGR-S) Part A was charged to a 2 liter reactor and brought to 40C. whileunder vigorous nitrogen purge. Eighteen milliliters of Part B was addedto the reactor followed by all of Part C. The remainder of Part B wasadded over the next 2.5 hours while the temperature was held at 39-41C.The latex was then digested at 40 for 1.5 hours, cooled to 30 andbottled. The latex contained 39.5 percent non-volatiles.

Four hundred grams of latex prepared above was mixed with g. of sodiumhydroxide dissolved in 120 g. of deionized water at 75C. to give a%-solution of polymer which was 31.4 percent by weight sodium acrylateand methacrylate. Ten grams of the above solution was mixed with 30 mg.of glycerine diglycidyl ether 1.0 percent curing agent by weight ofpolymer).

. 'A sheet was cast on mirror-finish chrome plate using a 25 mil drawbar, air dried, and oven cured for 15.5 hours at and 0.5 hour at Thispolymer sheet imbibed 23 times its own weight of synthetic urine (0.27 NNaCl solution) in the above absorbency test.

EXAMPLES 2-7 The procedure of Example 1 was repeated with varyingamounts of sodium hydroxide and curing agent. The results are shown inTable 111 along with Example E. A. Ethyl acrylate Hydroxyethyl acrylateis present in the amount of 8 mole 'r; in all examples.

*Glycerinc diglycidyl ether 1 In g. 0.27 N NaCl per g. polymer Theseexamples showthat softness and absorbency have an inverse relationship.The best all around product of these examples is one containingapproximately 50 mole residual acrylic esters and cross-linked byapproximately 0.15 weight glycerine diglycidyl ether.

EXAMPLE 8 Example 1 was repeated using the following monomer mix, andless initiator and no mercaptan chain stopper in order to raise themolecular weight.

The polymerization was carried out at 60C. and produced a latex of 40.6percent non-volatiles.

1125 g. of the above latex was added in a-small stream over a period of25 minutes to a slowly stirred solution of 187.16 g. 50% NaOH in 547.9g. deionized water. After the polymer had all dissolved, the viscoussolution was heated at 50C. for 22 hours to complete the saponification.The resulting solution (25.4 percent solids) had a Brookfield viscosityof 16,200 cps. at 25C. (No. 5 spindle, 10 rpm). The polymer is 50%ethylacrylate by moles with the remainder being sodium acrylate andmethacrylate.

32 g. of the above solution was blended with 16 mg. (0.2 wt.%) ofglycerine diglycidyl ether and cast on polished chromium plate with a 25mil draw bar. After air drying, the film was lifted from the plate andplaced in a oven. The absorbency (gel capacity) of the film in 0.27 NNaCl was 64 g. solution per gram polymer after a 20 minute cure. Thefilm was strong and flexible right out of the oven and required creasingin order to tear it.

EXAMPLES. 9-12 Eight grams of the saponified solution prepared inExample 8 was mixed with various amounts of 1,3- dichloroisopropanol andfilms were cast and cured from these mixtures in the manner of Example8. The gel capacities of the films are set forth in Table IV.

TABLE IV Wt.7( Mg. of D.C.|.P. Example D.C.I.P. Crosslinker Ahsorhenc v*D.(.l.P. 1.3-dichloruisopropanol gms 0.27 N NziCl per gm of polymerThese examples show the optimum level of crosslinker using D.C.l.P. isnear 0.5 percent by weight of polymer to be cured.

EXAMPLES l320 TABLE V Mole Wt.7( Example 71 E. A. G.D.E. AbsorbencySoftness 13 55 0.2 44.4 soft 14 52 0.2 48.8 soft 15 50 0.2 49.6 somewhatsoft 16 40 0.2 57.2 brittle 17 55 0.175 52.4 soft 18 52 0.175 58.8 soft19 50 0 175 58.4 somewhat soft 20 40 0.17 5 60.0 brittle E. A. ethylacrylate G.D.E. glycerine diglycidyl ether These examples show that theunconverted ethyl acrylate portion of the copolymer can be increased to52 mole without significant loss in absorbency and a desirably softproduct is obtained.

EXAMPLE 21 Three mixtures were made up having the followingcompositions:

Part A 230 gms deionized water 0.3 gms Triton GR-5 1.0 gms Na S O(sodium persulfate) 10.0 gms itaconic acid Part B 20 gms methacrylicacid 170 gms ethyl acrylate Fart 70 s deionized water -.Part A waschargedto a 2 liter reactor and brought to 60C. while under a vigorousnitrogen purge. Then 20 ml. of Part B was addedfollowed by all of PartC. The remainder of Part B was added continuously over a period of onehour at 60C. Thelatex was digested for one hour at this temperature toobtain a final latex having 40.6 percent non-volatiles.

Then, gms of the above latex was mixed with 16.9 gms of a 50'percentaqueous sodium hydroxide solution and 49.1 gms of deionized water andheated at 55C. for about 10 hours to give a 25 percent solution ofpolyelectrolyte containing 52 molar percent ethyl acrylate. Thispolyelectrolyte also was calculated to have 51% by weight ethyl acrylate30% by weight sodium acrylate 12.3% by weight sodium methacrylate 6.5%by weight disodium itaconate.

Twenty grams of the above polyelectrolyte was blended with 45 gramswater and 7.5 mg. (0.15%) glycerine diglycidyl ether. A film was cast ona mirror finished chrome plate using a 25 mil drawbar and this film wasair dried 6.5 hours at room temperature and then oven cured at C. for16.5 hours. The absorbency of the final cured film was 41 gms/gms ofpolymer in the above absorbency test with synthetic urine.

Example 21 illustrates that the polymerization recipe and process can besignificantly altered but still allow the preparation of highlyabsorbent polymer.

We claim:

1. A method for making a crosslinkable aqueous solution which is usefulto form soft water-swellable polyacrylate articles which comprises A.forming-a polyacrylate solution having about 30 to about 70 weightpercent alkali metal carboxylates by dissolving in an aqueous alkalimetal hydroxide solution containing sufficient alkali metal hydroxide tosaponify some of the acrylate esters and to neutralize the carboxylicacid groups a polyacrylate comprising I. about 30 to about 92 percent byweight of an alkyl acrylate wherein the alkyl group has l-10 carbonatoms, an alkyl methacrylate wherein the alkyl group has 4-10 carbonatoms, or mixtures thereof,

2. about 8 to about 50 percent by weight of an olefinically unsaturatedcarboxylic acid, and

3. about 0 to about 15 percent by weight of an omega hydroxy-alkylacrylate having 1-4 carbon atoms in the hydroxy alkyl group B. heatingthe solution until saponification is complete, and

C. adding to said solution about 0.1 to about 10 weight percent, basedon the dissolved polymer, of a water soluble crosslinking agent which isreactive with carboxylate salt groups.

2. The method as set forth in claim 1 wherein the unsaturated carboxylicacid is a poly carboxylic acid.

3. The method asset forth in claim 2 wherein the polycarboxylic acid isselected from maleic, fumaric, itaconic, aconitic, teraconic,citraconic, mesaconic, glutaconic or mixtures thereof.

4. The methodv as set forth in claim 1 wherein said unsaturatedcarboxylic acid is a monocarboxylic acid.

5. The method as set forth in claim 4 wherein the monocarboxylic acid isselected from acrylic. methacrylic, crotonic, isocrotonic, angelic,tiglic, senecioic, or mixtures thereof.

acrylic acid.

8. The method as set forth in claim 6 wherein the water solublecrosslinking agent is a polyglycidyl ether. 9. The method as set forthin claim 8 wherein the 5 polyglycidyl ether is glycerine diglycidylether.

1 UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT N0. 3,926,891 DATED December 16, 1975 INVENT0R(5) James R. Grosset a1.

It is certified that error appears in the aboveidentified patent andthat said Letters Patent is hereby corrected as shown below:

Q Column 2, line 38, "50 to 92" should read 30 b0 92 Column 8, line +6,8 to about 50" should read 8 to about '70 Signed and Scaled thisEleventh Day of December I979 [SEALE Arrest:

I SIDNEY A. DIAMOND Attesting Oflicer Commissioner of Patents andTrademarks

1. ABOUT 30 TO ABOUT 92 PERCENT BY WEIGHT OF AN ALKYL ACRYLATE WHEREINTHE ALKYL GROUP HAS 1-10 CARBON ATOMS, AND ALKYL METHACRYLATE WHEREINTHE ALKYL GROUP HAS 4-10 CARBON ATOMS, OR MIXTURES THEREOF,
 1. A METHODFOR MAKING CROSSLINKABLE AQUEOUS SOLUTION WHICH IS USEFUL TO FORM SOFTWATER-SWELLABLE POLYACRYLATE ARTICLES WHICH COMPRISES A. FORMING APOLYACRYLATE SOLUTION HAVING ABOUT 30 TO ABOUT 70 WEIGHT PERCENT ALKALIMETAL CARBOXYLATES BY DISSOLVING IN AN AQUEOUS ALKALI METAL HYDROXIDESOLUTION CONTAINING SUFFICIENT ALKALI METAL HYDROXIDE TO SAPONIFY SOMEOF THE ACRYLATE ESTERS AND TO NEUTRALIZE THE CARBOXYLIC ACID GROUPS APOLYACRYLATE COMPRISING
 2. about 8 to about 50 percent by weight of anolefinically unsaturated carboxylic acid, and
 2. ABOUT 8 TO ABOUT 50PERCENT BY WEIGHT OF AN OLEFINICALLY UNSATURATED CARBOXYLIC ACID, AND 2.The method as set forth in claim 1 wherein the unsaturated carboxylicacid is a poly carboxylic acid.
 3. about 0 to about 15 percent by weightof an omega hydroxy-alkyl acrylate having 1-4 carbon atoms in thehydroxy alkyl group B. heating the solution until saponification iscomplete, and C. adding to said solution about 0.1 to about 10 weightpercent, based on the dissolved polymer, of a water soluble crosslinkingagent which is reactive with carboxylate salt groups.
 3. The method asset forth in claim 2 wherein the polycarboxylic acid is selected frommaleic, fumaric, itaconic, aconitic, teraconic, citraconic, mesaconic,glutaconic or mixtures thereof.
 3. ABOUT 0 TO ABOUT 15 PERCENT BY WEIGHTOF AN OMEGA HYDROXY-ALKYL ACRYLATE HAVING 1-4 CARBON ATOMS IN THEHYDROXY ALKYL GROUP B. HEATING THE SOLUTION UNTIL SAPONIFICATION ISCOMPLETE, AND C. ADDING TO SAID SOLUTION ABOUT 0.1 TO ABOUT 10 WEIGHTPERCENT, BASED ON THE DISSOLVED POLYMER, OF A WATER SOLUBLE CROSSLINKINGAGENT WHICH IS REACTIVE WITH CARBOXYLATE SALT GROUPS.
 4. The method asset forth in claim 1 wherein said unsaturated carboxylic acid is amonocarboxylic acid.
 5. The method as set forth in claim 4 wherein themonocarboxylic acid is selected from acrylic, methacrylic, crotonic,isocrotonic, angelic, tiglic, senecioic, or mixtures thereof.
 6. Themethod as set forth in claim 1 wherein the water soluble crosslinkingagent is selected from polyhaloalkanols, sulfonium zwitterions,haloepoxyalkanes, polyglycidyl ethers, and mixtures thereof.
 7. Themethod as set forth in claim 1 wherein the polyacrylate which isdissolved comprises 85 weight percent ethyl acrylate and 15 weightpercent methacrylic acid.
 8. The method as set forth in claim 6 whereinthe water soluble crosslinking agent is a polyglycidyl ether.
 9. Themethod as set forth in claim 8 wherein the polyglycidyl ether isglycerine diglycidyl ether.